Some nonspecific spoilers here.
Expected a B horror film quality but was pleasantly surprised. I think the acting & detail to the time period visually was very solid. Maybe the date was off a bit. So let's change the line to "killed alot of Buffalo in that area " instead of 'All'. Maybe she misspoke or felt strongly about it. Still big trouble for an already limited pop of creatures evolved to a very specific area, who can't travel fast ,far, or at all in daylight.
Another view is to say it was entirely possible it was just slang back then to say any small groupings of displaced natives were called 'reservations' all over to many uneducated racist soldiers of the time.Then the history facts people wanted to teach us in most of these reviews are taken care of. Yay.
It was hard to easily pick out the 'good guys' & all of them at one point or another could be very deeply grey if not calloused to the point of ignoring evil acts. Some seemed like likeable people until things got bad & others starting out hostile surprised you. That made it feel like no one was safe & they were not. It could be the D&D player in me ,but I like that dynamic in a group.
Also, didn't like how clumsy the creatures seemed when revealed, but really they were burrowing creatures. Not evil ,but animalistic ambush predators. They were very tough & specialized to take down or pursue the much larger & stronger Buffalo with poison & tracking kind of like slow Komodo Dragons but combined creepy spider-like feeding. Horrifying way to go. The native woman was right that if you kept moving they couldn't keep up. I was glad they were just a bit strong for digging, but not impossibly fast underground with impossible tunnel systems like movies go with similar monsters. I think the Native hunters at the end set their trap on the edge of a wood so none of the creatures could easily burrow up close due to tangled roots therefore exposing themselves.
Lastly ,it was total karma that uncaring depopulation of wildlife caused this. Also the evil/racism of some soldiers didn't just feed the hate cycle with the natives ,but in the end may have allowed this other ancient danger to survive making the world an even worse place than they were succeeding at already.
Not too scary or any jump scares at all. Creepy & great atmosphere created. A cool monster hunt if you like those but action fans may get bored. Alot of realism in the human element. Could've used a little humor but realistically makes sense here,so much so that it is depressing at times. Overall it made me think & I enjoyed it & that is saying something for a B horror movie.